As a method for controlling an image processing characteristic of image forming apparatus such as a copier or a printer (referred to as an “image control method”), the following method is known.
An image forming apparatus is started, and after the completion of warm-up operation, a particular pattern is formed on an image holding member such as an electrostatic drum. Then, the density of the formed pattern is read, and the operation of circuit to determine an image forming condition such as a gamma corrector is changed based on the read density value, thus the quality of formed image is stabled.
Further, in a case where a tonality characteristic of the image forming apparatus is changed due to change of environmental condition, the particular pattern is again formed on the image holding member, then the pattern is read, and the read value is fed back to the circuit to determine the image forming condition such as a gamma corrector, thereby the image quality can be stabled in correspondence with change of environmental condition.
Further, in a case where the image forming apparatus is used for long time, the density of pattern on the image holding member may not correspond with that of actually print-outputted image. Accordingly, known is a method for forming a particular pattern on a recording medium and correcting an image forming condition based on the density value of the pattern.
Further, known is a method for converting a gamma look-up table (γGLUT) or generating a γLUT modulation table based on density information of one image pattern, and adding required correction information to a gamma corrector.
In the above methods, much time and troublesome work are required for the control, therefore image control cannot be performed frequently. Accordingly, it cannot be said that image quality such as tonality reproducibility is sufficiently stabilized in correspondence with constantly varying image characteristic of image forming apparatus. Further, in the method for correcting the γLUT by density information of one image pattern and adding correction information to the gamma corrector, which can correct the gamma corrector, comparatively easily, if the number of addition increases, the difference between gray scale levels of γLUT cannot be ignored, and pseudo outlines occur.